Synchronous motors are commonly used in industrial power applications to generate AC power. A synchronous AC induction motor is comprised of a stator and a rotor. The stator assembly is typically comprised of steel laminations shaped to form poles, with copper wires wound around these poles to form primary windings connected to a voltage source for producing a rotating magnetic field. A rotor assembly is typically comprised of laminations formed around a steel shaft core, with radial slots disposed around the laminations' periphery having rotor bars in the form of conductors shorted at the ends disposed parallel to the steel shaft core. Torque within the AC induction motor is developed by the interaction of currents induced in the rotor bars with the rotating magnetic field. Because of its inductive nature, a synchronous motor turning under a load will always turn at a rotational frequency less than that of the rotational frequency of the magnetic field, allowing the rotor bars to cut magnetic lines of force and produce useful torque. In addition, a synchronous motor is not by itself inherently capable of providing variable speed operation, with its speed determined by the frequency of the input power, the nature of the load and the current available. These operating characteristics of a synchronous motor are the result of a phenomenon known as “slip.”
With the speed of the synchronous motor determined by, among other things, the frequency of the input power provided to the motor, the output of a pulse width modulator (PWM) is commonly provided to a synchronous motor for providing the motor with a variable output frequency. The width of the pulses is controlled at the input to the synchronous motor and appears as a sinusoidal 3-phase signal. These pulse width modulated signals are typically generated by means of a digital signal processor. This process involves the calculation of the specific pulse width at much faster speeds than the output signal frequency. A large load on these signal processors limits the output frequency of current synchronous motors to on the order of 3 kHz. Employing multiple processors to provide drive signal inputs of increased frequency substantially increases the complexity, particularly of the control software, and the expense of the synchronous motor drive arrangement.
The present invention addresses the aforementioned limitations of the prior art by providing a multi-phase, multi-frequency controller for an AC synchronous motor using heterodyne signal conversion with controlled variable oscillator feedback technology.